Several cells have been described in lymphoid tissue which have long dendritic processes and are morphologically distinguishable from lymphocytes and typical macrophages. One such dendritic-type cell resides in the B-cell copartment of lymph nodes and spleen and is especially prominent in lymphoid follicles. A newly proposed system of nomenclature for dendritic-type cells (D-cells), recommends the name "follicular dendritic cell" and the abbreviation "FDC" for this D-cell. (Tew, Thobecke, and Steinman, 1982). The cardinal feature of this cell, which distinguishes it from other leukocytes including other D-cells, is its ability to trap and retain immune complexes for extended periods of time (e.g. T 1/2 of HSA on mouse FDCs is approximately 2 months (Tew, and Mandel, 1979). Although this cell has been observed in sections of lymphoid tissues and its morphology described in a number of studies, its immune functions remain obscure. In part this may be attributable to the fact that this cell is very large, fragile and, in work up to the present application, has never been clearly identified in vitro. Nevertheless data from our laboratory support the concept that antigen persisting in association with FDCs functions in an antibody feedback system to maintain and regulate serum antibody levels in vivo. Data supporting this hypothesis were recently summarized in Immunological Reviews (Tew, Phipps, and Mandel, 1980). In addition, it has been proposed that FDCs play a major role in the induction of B-cell memory. Data supporting this hypothesis were also recently reviewed (Klaus, et al., 1980). Our long term research objective is to thoroughly test these hypotheses and establish whether antigen persisting on FDCs plays the proposed roles in the maintenance of specific antibody levels and B-cell memory. We have now established methods for releasing intact FDCs fdrom lymph nodes and methods for enriching and identifying FDCs in culture. We are now proposing to carry out a second generation of experiments using isolated FDCs in vitro as well as in vivo. In addition, we propose to characterize FDCs with respect to their cell surface markers, properties in cell culture, antigen trapping ability, etc. We also propose to determine if FDCs are of bone marrow origin, document stages of FDC in differentiation and determine their role in stimulation B-cells and in germinal center formation.